Good evening all.
Been lurking for a while, so my first comment is - Frosty and Mikey, I already have. =D
Reading AFB's and DRottBlatt's discussion on the trumpet/bell reminded me of some older YouTube videos put out by a turbine engine repair mechanic. He explains why they install a bell mouth on the static test stand.
https://www.youtube.com/watch?v=3DQdcvzf9RM starts at 6:00
https://www.youtube.com/watch?v=eB3f_Idkiow starts at 7:45
Good luck gents,
Brinton